# Using Python to calculate northern hemisphere’s surface land coverage

Yesterday during my lunch break I was rather bored; it is unseasonably cold for the fall, even in Calgary, and a bit foggy too.
For something to do I browsed the Earth Science beta on Stack Exchange looking for interesting questions (as an aside, I encourage readers to look at the unanswered questions).
There was one that piqued my curiosity, “In the northern hemisphere only, what percentage of the surface is land?”.
It occurred to me that I could get together an answer using an equal area projection map and a few lines of Python code; and indeed in 15 minutes I whipped-up this workflow:

• Invert and import this B/W image of equal area projection (Peters) for the Northern hemisphere (land = white pixels).

Source of original image (full globe): Wikimedia Commons

• Store the image as a Numpy array.
• Calculate the total number of pixels in the image array (black + white).
• Calculate the total number of white pixels (1s) by summing the entire array. Black pixels (0s) will not contribute.
• Calculate percentage of white pixels.

The result I got is 40.44%. Here’s the code:

```# import libraries
import numpy as np
from skimage import io
from matplotlib import pyplot as plt

# import image

# check the image
fig = plt.figure(figsize=(20, 10))
ax.set_xticks([])
ax.set_yticks([])
plt.imshow(north_equal_area, cmap = 'gray');

# Do the calculations
r, c = np.shape(north_equal_area)
sz =  r*c
s = np.sum(north_equal_area)
print(np.round(s/sz*100, decimals=2))
>>> 40.44```

As suggested in a comment to my initial answer, I run the same Python script for the entire globe and got the expected 30% land coverage:

```# import image

# Do the calculations
r1, c1= np.shape(equal_area)
sz1 =  r1*c1
s1 = np.sum(equal_area)
print(np.round(s1/sz1*100, decimals=2))
>>> 30.08
```

# Do you know any cool apps?

I’d like to pick up my Apps page, which I sort of abandoned a while back.

If you have any great app to recommend, I’d love to hear about it so please add them in the comment section to this post. I am looking for Apps for Android and iPhone/iPad in the following categories – ideally free or very low-cost, possibly open-source:

Geology

Geophysics

Cartography and mapping

Planetary Science

Image Processing

Visualization

# Color Use Guidelines for Mapping and Visualization

I find the Color Use Guidelines by Cynthia Brewer (of Color Brewer fame) very well done and extremely useful. Below is a screen captures of the main page. To see an explanation of and example for each color scheme visit the interactive guidelines.

# colour maps

The perfect lead into my series on perceptual color palettes. Great post!

The original article on the Guardian is here. And here is the conversation that lead to improved map, as put together on Storify.

I thought it’d be interesting to run a simulation of what the map would actually look lie to viewers with the 3 types of color deficient vision. Below are my results for the first map. It is obvious from this simulation that while the map is OK for Tritanope viewers, the green and red areas are very confusing for Protanope and Deuteranope viewers.